1. Field of the Invention
The present invention relates to a method of bonding a transparent substrate and a substrate together with a liquid adhesive in a manner that brings the two into full-surface contact with each other. For example, the present invention relates to bonding a transparent cover plate of a cellular phone or other similar devices to a flat display such as a liquid crystal display device, a plasma display, an organic EL display, an inorganic EL display, or an FED, or bonding a touch panel to a flat display.
2. Description of the Related Art
Conventionally, a general method to place a transparent touch panel on a display device has been to fix the transparent touch panel to the perimeter of the display device by providing cushion rubber with adhesive about 0.5 mm or more in thickness. Another method is to bond the liquid crystal display device and the touch panel together in full-surface contact with the use of a transparent adhesive (see, for example, JP 09-274536 A, which is hereinafter referred to as Patent Document 1).
Still another known method uses a transparent adhesive sheet to bond the touch panel and the display panel together (see, for example, JP 2004-5540 A, which is hereinafter referred to as Patent Document 2). An advantage of full-surface contact bonding with the use of the transparent adhesive sheet is that the adhesion of one side surface of the transparent adhesive sheet can be adjusted such that, if air bubbles, foreign objects, and the like are caught in-between during bonding, the sheet is peeled off and stuck again after cleaning. In cellular phones, a transparent cover plate is placed over a liquid crystal display device, and a black-colored member or the like is formed by printing in the perimeter of the transparent cover plate outside a display area of the liquid crystal display. Commonly, an elastic member made of rubber or the like is interposed between the printed member and the liquid crystal display outside the display area.
Another way to bond the display area in a full-surface contact manner is a sheet obtained by laminating a silicon gel layer and a silicone rubber layer (see, for example, JP 2004-101636, which is hereinafter referred to as Patent Document 3).
Recently, demand has been growing for thinner cellular phones which measure 0.1 mm or less between the transparent cover plate and the liquid crystal display. Employed for the transparent cover plate is transparent plastic such as an acrylic or polycarbonate, glass, or the like. The transparent cover plate includes a low reflective film made up of layers of materials to vary the refractive index in stages, an electromagnetic shield formed of copper, aluminum, or the like and forming a grid-like etching pattern, and a hard coat for preventing scratches. In the case of a glass cover plate, a film sheet for preventing cracking, a film sheet subjected to anti-glare treatment to prevent specular reflection, or the like is stuck over the glass surface. The transparent cover plate and the flat display have a rectangular shape inmost cases. Variations of the touch panel include resistive panels (analog resistive film type and digital resistive film type), capacitive (CAP) panels, and surface acoustic wave (SAW) panels.
Air bubbles and foreign objects trapped during bonding are a serious issue common to all of various methods of bonding a liquid crystal display to a touch panel or a transparent cover plate that have been proposed. Although whether air bubbles and foreign objects are being trapped is checked by casting light from the screen side of the transparent cover plate or of the touch panel after bonding in the case of a transparent adhesive sheet, or before curing in the case of a transparent adhesive, there are following problems. The check may detect a foreign object but cannot tell whether the foreign object is in the bonded touch panel or transparent cover plate or in the bonded adhesive layer. It is very difficult to confirm the location of a defect in the touch panel or the transparent cover plate before conducting the post-bonding check. Also, in the case of a photo-curing transparent adhesive, especially a visible-light curable adhesive, the light cast for the check accelerates the curing of the adhesive to a point that makes it difficult to peel when a defect is found, unless the check is finished quickly.